US3551247A - Laminated vacuum coated titanium structural material - Google Patents

Description

Dec. 29, 1970 F FEAKES 3,551,247

LAMINATED VACUUM COATED TITANIUM STRUCTURAL MATERIAL Filed Jan. 29, 1968 I, Q A" 42 g 423 United States Patent 3,551,247 LAMINATED VACUUM COATED TITANIUM STRUCTURAL MATERIAL Frank Feakes, Lexington, Mass, assignor to Norton Research Corporation, Cambridge, Mass. Filed Jan. 29, 1968, Ser. No. 701,385 Int. Cl. B32b /20; C23c 7/00 U.S. Cl. 156278 7 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION The present invention relates to a coated titanium product, a coated refractory metal product, composites with high modulus reinforcement, laminates and structural materials.

It is a principal object of the invention to provide a coated titanium or like refractory metal product of high modulus of elasticity and strength.

It is a further object of this invention to provide an improved form of titanium structural material characterized by higher modulus of elasticity than titanium and a lower density to be used for any given application where high stiffness and low weight are required.

The object is achieved through the use of a laminate approach in which titanium in the form of foil about half mil thick is coated with boron, a high modulus of elasticity material, and laid up in laminates. Alternatively, the foil is coated with boron carbide. In order to achieve effective amounts of boron or boron carbide in the laminate, it is coated in a thickness of at least one-fourth as great as that of the substrate and preferably in a thickness greater than half the substrate thickness. The combination of titanium and boron results in a surprisingly low internala stress despite the significant thickness of the coating relative to the substrate. In order to achieve the necessary adhesion between the coating and the substrate at relatively low temperature an aluminum undercoat is deposited on the titanium prior to coating with boron. It is surprising that adherent thick coatings of boron or boron carbide could be obtained on titanium at relatively low temperatures, of vacuum deposition.

Another distinct aspect of the invention is the discovery that a way to achieve a useful and practical form of the known high-modulus-at-low-density materialsboron and boron carbideis to apply them as thick coatings onto a titanium substrate. This involves accepting the apparent weight penalty of titanium in return for the surprisingly more-than-oifsetting gains of achievable thick and adherent coatings of the high modulus material. The validity of this approach to coated product reinforcing structures has been demonstrated.

In making the laminate, the layers of coated product can be bonded by adhesive or self-bonded by hot pressing.

It is a further object of the invention to provide a method of coating boron and boron carbide onto titanium, using an interlayer.

It is possible to coat boron or boron carbide directly on titanium. But, such coating requires uniform heat ing of the coating area to a temperature of about 500 C.- 800 C. to insure good adhesion. Uniform heating of thin titanium substrates is not a simple processing task and the difliculty is greater when done in Vacuum. In addi- 3,551 ,247 Patented Dec. 29, 1970 tion, high coating temperatures increase the effects of small differences in the thermal expansion coefficients of the substrate and coating. The stresses produced by these differences in coefficients of expansion may limit the coating thickness of boron and boron carbide attainable on a practical basis at SOD-800 C. The small difference of thermal expansion between titanium and boron and between titanium and boron carbide due to thermal gradients across a thickly coated product and/or small differences in coefficient of thermal expansion, both accentuated at high processing temperature limits the coating thickness of boron or boron carbide attainable on a practical basis at 500 C.-800 C. The coating of boron or boron carbide on an aluminum coated face of a titanium substrate in accordance with the present invention can be carried out at about 200300 C. for boron and for boron carbide. Consequently, substantially greater coating thicknesses may be attained. The attainable thickness is also increased by the closer match of titanium to boron and boron carbide in coefiicient of thermal expansion compared to other substrates. This improvement in attainable thickness and hence of volume fraction of boron or boron carbide which can be introduced into a composite offsets the weight penalty of titanium compared to lighter available substrates (plastic, aluminum). Titanium also lends desirable strength to the structural laminate as a whole.

The coeflicient of thermal expansion of boron carbide is less than that of titanium. However by increasing the volume fraction of aluminum, the coefficient of the combination of aluminum and boron carbide may be made equal to that of titanium. Under these circumstances the aluminum will yield and the combination of coating and substrate will remain flat as deposited.

The invention is now further described in the following specific example with reference to the accompanying drawings wherein:

FIG. 1 is a schematic diagram of the apparatus used in practicing the method of the invention,

FIG. 2 is a schematic cross-section view of a structural laminate incorporating as a repetitive building block, the coated product of the invention.

Referring to FIG. 1, there is shown a vacuum chamber 10 evacuated by a vacuum pumping system 12. Within the chamber is mounted an evaporation source 14 such as a water-cooled copper crucible filled with a charge of the material to be evaporated (boron or boron carbide), an electron beamgun 16 for heating the charge with a beam of electrons and a shutter 18 (movable to the position indicated at 18 to allow coating). The substrate to be coated is indicated at 20 and is moving from a feed roll 22 to a wind up roll 24 passing over wheels 26. The substrate also passes over a cooled back-up plate 28, the temperature of which is controlled by circulation of a coolant in pipes 29.

Heater rods 30 are provided to preheat the substrate to the temperature necessary for good adhesion. The rods are electrical resistance heating elements arranged in pairs with current flow in opposite directions in the adjacent rods of each pair so that there is no net significant magnetic field to disturb the beam from electron gun 16 to source 14.

The substrate comprises an aluminum coated titanium sheet in a distinctly advantageous embodiment of the invention. The aluminum is evaporated in vacuum by any of electron beam, resistance or induction heating and deposited on a clean titanium foil moving past the evaporating source to produce a coating of aluminum. Where the final coating is to be boron, the thickness of the aluminum coating will be a mere flash coating thickness on the order of about 10 microinches.

Where the desired final coating is boron carbide, the aluminum coating is deposited to a thickness of at least the desired boron carbide thickness but no greater than half the desired boron carbide thickness. This is to help buffer the effects of slight mismatch in the thermal expansion coefficients of boron carbide and titanium (4 and 8, respectively) without getting the aluminum so thick that the effective substrate becomes aluminum rather than titanium.

Prior to the aluminum precoating, the titanium is preheated in vacuum for cleaning, to a temperature at least as high or higher than any subsequent temperature which will be encountered in the later steps of aluminum precoating or boron (or boron carbide) final coating.

It is distinctly advantageous that the temperature of the titanium foil during the pre-heating, precoating and coating steps be held below the temperature of its alpha/ beta transition which is l62l F. (885 C.) for pure titanium, about 1650 F. for high titanium alloys, 1665 F. for low titanium alloys and 1750 F. for the common Ti-6Al-4V high strength alloy, particularly during the latter two steps. Internal stresses are thus minimized and thicker adherent coats of boron or born carbide are made more readily by eliminating this source of stress as well as the stresses associated with thermal expansion per se.

The boron or boron carbide charge is degassed in a vacuum on the order of l0 torr by heating at 1300 C. for 2 hours prior to coating. In the case of boron a further preparation step of deep melting by electron bombardment in vacuum is carried out prior to coating.

A preferred heating schedule for the titanium is 600 C. for preheating 350 C. for aluminum precoating and 250 C. for final coating.

The coated product of the invention is shown in FIG. 2, as incorporated in a structural laminate. The laminate 40 comprises multiple layers of coated titanium, only two of such layers 42 and 44 being shown in the drawing. The layers are secured to each other by hot pressed diffusion bonding (pressure of 2000 p.s.i., temperature of 600 C. applied for 120 minutes), assisted by use of a flash vacuum coating of aluminum 43 coated on the back face of the titanium substrate 441 of layer 44. The layer 42, which is typical of the coated product of the invention, comprises a substrate 421 of .0005 inch titanium foil. The coating 422 is a vacuum deposited layer of boron .005 inch thick secured to the substrate by a vacuum deposited coating 423 of aluminum .00005 inch thick and the diffusion reaction products of aluminum and titanium in layer 424 and of aluminum and boron in layer 425. The structural material 40 may be employed as a simple fiat wall member or curved as in an airplane fairing. Curved shapes can be laid of individual coated layers and hot pressed in curved form to produce a final curved product which is quite rigid. The laminating can also be produced by methods other than metal assisted hot pressing, such as self-bonded hot pressing, epoxy bonding by painting one or both of mating surfaces within the laminate and holding the laminate under conditions to promote curing of the epoxy adhesive. The use of an all metal structural material is preferred, over one having epoxy or other resins, in order to facilitate high temperature use such as in hot gas turbine blades.

Another embodiment of the invention involves production of the structural laminate material of the invention by forming the titanium layers in situ by consecutive deposition. For instance, an aluminum core in the shape of a turbine blade could be rotated in a vacuum chamber over three sources of titanium, boron and aluminum which could deposit in sequence layers of (Ti-Al-B -Al- (Ti-Al-B -Al etc., the B and Ti layers being .O000l inch thick and the Al layers being .000001 inch thick. The temperature at the deposition surface would be held to between 100 C.

4 and 500 C. and preferably about 300 C. by a combination of heaters and radiant heat sinks adjacent the deposition zone.

As further variations of the invention the aluminum metal precoat can be substituted by other metals which adhere well to the substrate and final coat under vacuum coating conditions, such as nickel, magnesium, zinc, lead and cadmium. The substrate can be substituted by alloys of titanium and by other refractory metals-columbium, zirconium, hafnium, tantalum, tungsten, molybdenum and alloys thereof. All the foregoing have coefficients of thermal expansion in the range of 4 to 8 10" C.

(In/in. C.) Columbium 8.0 X l0 Zirconium 5 X 10 Hafnium 6 X 10 Tantalum 6.7 X 10* Tungsten 4.6 x10- Molybdenum 5.5 x 1O- which is essentially the same as 4 for boron carbide and 8 for boron for purposes of the present invention and in contrast to aluminum and plastic substrates whose coefficients are 25 l0 C. and up. The use of the above substitutions with the present advantages is limited by the constraint that the precoated substrate must be capable of being coated by the final coat material at temperatures below those involved in coating without benefit of the precoat to limit stresses as described above. It is also preferable that the precoat be coatable on the substrate at relatively low temperatures.

The following examples of practice of the invention are now set forth by way of illustration and not by way of limitation.

EXAMPLE 1 Substrate in the form of titanium foil, 0.5 mil thick and coated with vacuum deposited aluminum on both sides to a thickness of .07 mil per side was cut into several strips (typical dimensions 2 by 10 inches and 6 by 12 inches) and mounted on a 2 mil thick carrier web of aluminum. The substrate was coated with boron, in the apparatus of FIG. 1, with an aluminum coated side facing downwardly towards the boron source. This coating was carried out under conditions of about l0 torr pressure, l030 kilowatts to the electron beam gun, substrate speed of .l.6 foot per minute, coolant temperatures of about 160 C., preheating the substrate to temperatures of about 200 to 300 C. with power on the order of 1-3.5 kilowatts fed to the preheater rods.

After coating, the titanium foils were separated from the carrier and found to have boron contents of 48% by volume in two separate runs.

The various coated foils were formed into laminates using epoxy resin adhesive between layers, the epoxy forming 15 to 30% of the laminate. One laminate had a volume fraction of 34% boron and the other 42%. The laminates were tested for strength in tension along a single axis. The laminate made with 34% boron had an elastic modulus of about 20x10 p.s.i. and the 42% boron laminate had an elastic modulus of 28 10- p.s.i.

EXAMPLE 2 A substrate in the form of titanium foil (0.5 mil substrate thickness) was coated in a stationary system in vacuum with a layer of aluminum approximately 0.04 mil thick. This, in turn, was coated with boron carbide at a pressure of between l l0 and 1X10" torr. The substrate temperature was maintained between 400 and 600 C. The thickness of the boron carbide deposit was approximately 0.2 mil. The power to the electron gun was 2 kilowatts.

5 EXAMPLE 3 In order to carry out consecutive deposition, the above process of Example 2 was repeated by depositing a second layer of aluminum on the first boron carbide layer. The second aluminum layer was then followed by a second layer of boron carbide.

The cooled substrate was then removed from the vacuum system and cut into four strips. These four strips were then bonded together using an epoxy adhesive (Union Carbide EAL 2256). The resulting laminate was tested in tension. The tensile modulus of elasticity was 31.0 10- p.s.i. The proportional limit was 54,700 p.s.i. and the ultimate strength 54,700 p.s.i. The make-up of the laminate was 34 volume percent B4C, 6 volume percent aluminum, 40 volume percent titanium and 20 volume percent epoxy.

What is claimed is:

1. A method of producing a basic repetitive unit for a high modulus of elasticity laminate product comprising the steps of (a) placing a sheet form refractory metal substrate layer under vacuum,

(b) vacuum depositing a thin layer of an undercoat metal on at least one surface of said sheet form metal,

(0) vacuum depositing a thick layer of a high modulus of elasticity material on an undercoated surface of said sheet form substrate,

said high modulus of elasticity material being selected from the class consisting of boron and boron carbide, the refractory metal substrate being selected from the class consisting of elemental and alloyed titanium, columbium, zirconium, hafnium, tantalum, tungsten, and molybdenum metals.

2. The method of claim 1 wherein the substrate is titanium. Y i

3. The method of claim 2 wherein the undercoat metal is aluminum.

4. The method of claim 3 wherein the substrate is heated to a temperature of at least 200 C. and no greater than 300 C. during vacuum deposition of the high modulus material thereon.

5. The method of claim 1 wherein a laminate is produced by assembling and bonding together said repeating units.

6. The method of claim 5 wherein additional layers of said undercoat metal are applied to the repeating units as adhesive.

7. The method of claim 1 wherein the laminate is formed by consecutive deposition of the repeating units.

References Cited UNITED STATES PATENTS 2,833,668 5/1958 Dailey et al 117105X 2,945,779 7/1960 Lipinski 161l86X 2,957,794 10/1960 Shetterly et al. 161l86X 2,996,412 8/ 1961 Alexander 117221 3,102,044 8/1963 Joseph 117119X 3,306,764 2/1967 Lewis et al 161-182X 3,348,967 10/1967 Hucke 161213X 3,367,826 2/1968 Heestand et a1. 161--182 3,372,105 3/1968 Johnson 161182X 3,460,976 8/1969 Allen 117l69X 3,475,161 10/1969 Ramirez 117131X 3,476,586 11/1969 Valtchev et al 117-105X 3,496,621 2/1970 Winter 29194X HAROLD ANSHER, Primary Examiner US. Cl. X.R.

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